An electronic device may be created from a substrate that has undergone various processes. One of these processes may include introducing impurities or dopants to alter the electrical properties of the original substrate. For example, charged ions, as impurities or dopants, may be introduced to a substrate, such as a silicon wafer, to alter electrical properties of the substrate. One of the processes that introduces impurities to the substrate may be an ion implantation process.
An ion implanter is used to perform ion implantation or other modification of a substrate. A block diagram of a conventional ion implanter is shown in FIG. 1. The conventional ion implanter may comprise an ion source 102 that may be biased by a power supply 101. The system may be controller by controller 120. The operator communicates with the controller 120 via user interface system 122. The ion source 102 is typically contained in a vacuum chamber known as a source housing (not shown). The ion implanter system 100 may also comprise a series of beam-line components through which ions 10 pass. The series of beam-line components may include, for example, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet collimator 110, and a second deceleration (D2) stage 112. Much like a series of optical lenses that manipulate a light beam, the beam-line components can manipulate and focus the ion beam 10 before steering it towards a substrate or wafer 114, which is disposed on a substrate support 116.
In operation, a substrate handling robot (not shown) disposes the substrate 114 on the substrate support 116 that can be moved in one or more dimensions (e.g., translate, rotate, and tilt) by an apparatus, sometimes referred to as a “roplat” (not shown). Meanwhile, ions are generated in the ion source 102 and extracted by the extraction electrodes 104. The extracted ions 10 travel in a beam-like state along the beam-line components and implanted on the substrate 114. After implanting ions is completed, the substrate handling robot may remove the substrate 114 from the substrate support 116 and from the ion implanter 100.
Referring to FIGS. 2A and 2B, there is shown a block diagram illustrating the workpiece support 116 supporting the substrate 114 during the ion implantation process. As illustrated in FIG. 2A, the workpiece support 116 may comprise a sealing ring 202 and a plurality of embossments 204 that are in contact with the substrate 114. The sealing ring may be an annular ring of about 0.25 inches in width, and having a height of 5 microns. The embossments 204 may be about 1 mil in diameter and 5 microns high. In addition, the workpiece support 116 may also include at least one cooling region 206. During the implantation process, cooling gas may be provided to the cooling region 206 prevent the substrate 114 from overheating. The workpiece support 116 may have gas channels and conduits to allow this cooling gas to flow to the cooling region 206. The workpiece support 116 may further include a plurality of lift pins 208 that may move so as to push the substrate 114 away from the workpiece support 116 in the direction indicated by the arrows. The lift pins 208 may be retracted within the workpiece support 116, as illustrated in FIG. 2B. The workpiece would also be normally in contact with a plurality of ground pins 205.
The workpiece support 116 may be cylindrical in shape, such that its top surface is circular, so as to hold a disc-shaped substrate. Of course, other shapes are possible. To effectively hold the substrate 114 in place, most workpiece supports typically use electrostatic force. By creating a strong electrostatic force on the upper side of the workpiece support 116, the support can serve as the electrostatic clamp or chuck, the substrate 114 can be held in place without any mechanical fastening devices. This minimizes contamination, avoids wafer damage from mechanical clamping and also improves cycle time, since the substrate does not need to be unfastened after it has been implanted. These clamps typically use one of two types of force to hold the substrate in place: coulombic or Johnson-Rahbeck force.
As seen in FIG. 2A, the clamp 116 traditionally consists of several layers. The first, or top, layer 210, which contacts the substrate 114, is made of an electrically insulating or semiconducting material, such as alumina, since it must produce the electrostatic field without creating a short circuit. In some embodiments, this layer is about 4 mils thick. For those embodiments using coulombic force, the resistivity of the top layer 210, which is typically formed using crystalline and amorphous dielectric materials, is typically greater than 1014 Ω-cm. For those embodiments utilizing Johnsen-Rahbeck force, the volume resistivity of the top layer, which is formed from a semiconducting material, is typically in the range of 1010 to 1012 Ω-cm. The term “non-conductive” is used to describe materials in either of these ranges, and suitable for creating either type of force. The coulombic force can be generated by an alternating voltage (AC) or by a constant voltage (DC) supply.
Directly below this layer is a conductive layer 212, which contains the electrodes that create the electrostatic field. This conductive layer 212 is made using electrically conductive materials, such as silver. Patterns are created in this layer, much like are done in a printed circuit board to create the desired electrode shapes and sizes. Below this conductive layer 212 is a second insulating layer 214, which is used to separate the conductive layer 212 from the lower portion 220.
The lower portion 220 is preferably made from metal or metal alloy with high thermal conductivity to maintain the overall temperature of the workpiece support 116 within an acceptable range. In many applications, aluminum is used for this lower portion 220.
Initially, the lift pins 208 are in a lowered position. The substrate handling robot 250 then moves a substrate 114 to a position above the workpiece support 116. The lift pins 208 may then be actuated to an elevated position (as shown in FIG. 2A) and may receive the substrate 114 from the substrate handling robot 250. Thereafter, the substrate handling robot 250 moves away from the workpiece support 116 and the lift pins 208 may recede into the workpiece support 116 such that the sealing ring 202 and the embossments 204 of the workpiece support 116 may be in contact with the substrate 114, as shown in FIG. 2B. The ground pins 205 are also normally in contact with the substrate 114. The implantation process may then be performed with the lift pins 208 in this recessed position. After the implantation process, the substrate 114 is unclamped from the workpiece support 116, having been held in place by electrostatic force. The lift pins 208 may then be extended into the elevated position, thereby elevating the substrate 114 and separating the substrate 114 from the edge 202 and the embossments 204 of the workpiece support 116, as shown in FIG. 2A. The lift pins 208 are either insulating or conductive and therefore may not remove any remaining charge from the substrate 114. The substrate handling robot 250 may then be disposed under the substrate 114, where it can retrieve the implanted substrate 114 at the elevated position. The lift pins 208 may then be lowered, and the robot 250 may then be actuated so as to remove the substrate 114 from the implanter.
A condition that can occur with a conventional ion implanter 100 may be found in the process of removing the substrate 114 from the workpiece support 116. After multiple cycles of clamping and unclamping a substrate 114 to a workpiece support 116, the side of the substrate 114 clamped to the workpiece support 116 may exhibit damage. This damage may be due to electrical discharge caused by electrostatic charge buildup on the substrate 114 and the top layer 210 of the workpiece support 116. The electrostatic charge may discharge (arc) to a ground pin 205 or directly to the surface of the workpiece support 116.
Previously, substrates 114 have been grounded via contact with metal lift pins 208 or ground pins 205. Substrates 114 also have been grounded previously using a plasma flood gun (PFG). Due to the brief contact time and small contact area between the lift pins 208 or ground pins 205 and the substrate 114 area containing the electrostatic charge, a condition can exist wherein the lift pins 208 and ground pins 205 do not effectively drain the electrostatic charge from the substrate 114. Accordingly, there is a need in the art for an improved electrostatic clamp that can remove charge.